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Date:   Wed, 22 Nov 2017 17:19:07 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Josef Bacik <josef@...icpanda.com>
Cc:     rostedt@...dmis.org, mingo@...hat.com, davem@...emloft.net,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        ast@...nel.org, kernel-team@...com, daniel@...earbox.net,
        linux-btrfs@...r.kernel.org
Subject: Re: [PATCH v7 0/4] Add the ability to do BPF directed error injection

On Wed, Nov 22, 2017 at 04:23:29PM -0500, Josef Bacik wrote:
> This is hopefully the final version, I've addressed the comment by Igno and
> added his Acks.
> 
> v6->v7:
> - moved the opt-in macro to bpf.h out of kprobes.h.

Thanks Josef!
All patches look great to me.
We'll probably take them all into bpf-next.git to start testing together
with other bpf changes and when net-next reopens will send them to Dave.
Then optionally can send pull-req for the first patch only to tip if Ingo
thinks that there can be conflicts with the work happening in parallel
on kprobe/x86 bits.
This way hopefully there will be no conflicts during the next merge window.
Makes sense?

> v5->v6:
> - add BPF_ALLOW_ERROR_INJECTION() tagging for functions that will support this
>   feature.  This way only functions that opt-in will be allowed to be
>   overridden.
> - added a btrfs patch to allow error injection for open_ctree() so that the bpf
>   sample actually works.
> 
> v4->v5:
> - disallow kprobe_override programs from being put in the prog map array so we
>   don't tail call into something we didn't check.  This allows us to make the
>   normal path still fast without a bunch of percpu operations.
> 
> v3->v4:
> - fix a build error found by kbuild test bot (I didn't wait long enough
>   apparently.)
> - Added a warning message as per Daniels suggestion.
> 
> v2->v3:
> - added a ->kprobe_override flag to bpf_prog.
> - added some sanity checks to disallow attaching bpf progs that have
>   ->kprobe_override set that aren't for ftrace kprobes.
> - added the trace_kprobe_ftrace helper to check if the trace_event_call is a
>   ftrace kprobe.
> - renamed bpf_kprobe_state to bpf_kprobe_override, fixed it so we only read this
>   value in the kprobe path, and thus only write to it if we're overriding or
>   clearing the override.
> 
> v1->v2:
> - moved things around to make sure that bpf_override_return could really only be
>   used for an ftrace kprobe.
> - killed the special return values from trace_call_bpf.
> - renamed pc_modified to bpf_kprobe_state so bpf_override_return could tell if
>   it was being called from an ftrace kprobe context.
> - reworked the logic in kprobe_perf_func to take advantage of bpf_kprobe_state.
> - updated the test as per Alexei's review.
> 
> - Original message -
> 
> A lot of our error paths are not well tested because we have no good way of
> injecting errors generically.  Some subystems (block, memory) have ways to
> inject errors, but they are random so it's hard to get reproduceable results.
> 
> With BPF we can add determinism to our error injection.  We can use kprobes and
> other things to verify we are injecting errors at the exact case we are trying
> to test.  This patch gives us the tool to actual do the error injection part.
> It is very simple, we just set the return value of the pt_regs we're given to
> whatever we provide, and then override the PC with a dummy function that simply
> returns.
> 
> Right now this only works on x86, but it would be simple enough to expand to
> other architectures.  Thanks,
> 
> Josef

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